Add attributes to the CSV file generated as an artifact

[gaurav]

One of the artifacts we generate (by using the LinkML csvgen) is a CSV file listing the entities in the model. It would be helpful if we could improve this CSV file by:

• Adding attributes in addition to entities (see requirements for csvgen linkml/linkml#165 (comment) for a possible description of a format that could be used to do this).
• Adding direct and indirect mappings (i.e. exact and close matches).